1. Field of the Invention
The present invention relates to a developing device capable of supplying developer of a uniform concentration with stability and an image-forming device, such as a photocopier or printer, equipped with this developing device.
2. Description of Related Art
An image-forming device well known in the art, such as a photocopier, forms an electrostatic latent image on an image-carrying member by irradiating light from a laser light source or a light-emitting diode (LED) onto the image-carrying member based on image data. The latent image is developed with toner and the developed image is subsequently transferred onto a sheet and fixed to the sheet with heat. The image-forming device develops electrostatic latent images formed on the image-carrying member into visible images using either a single-component developer or a double-component developer including a toner and carrier.
When using a double-component developer, a magnetic developing roller is provided around the image-carrying member and is configured of a plurality of fixed magnetic poles and a sleeve capable of rotating around the magnetic poles. The magnetic developing roller forms a magnetic brush by magnetically attracting developer. As the sleeve rotates, the magnetic brush is conveyed over the image-carrying member.
The magnetic brush deposits toner suited to the potential of the latent image formed on the image-carrying member to develop the latent image. After development, the developer is recovered and reused together with an external supply of toner for developing latent images.
For forming stable toner images, the developer used in this construction must be supplied in a uniform amount (having a uniform toner concentration) and must be maintained at a specific charge amount to be attracted to the potential of the latent images formed on the image-carrying member.
The concentration of the toner changes depending on distribution of the amount of toner consumed during development and newly supplied toner, and the charge amount of the toner changes depending on friction produced when the toner and carrier are mixed together. Therefore, the developing device must sufficiently agitate the developer composed of a toner and carrier to achieve a uniform distribution of toner concentration, and saturate the toner with the applied charge amount in order to produce stable toner images.
One such developing device disclosed in Japanese unexamined patent application publication No. 2001-249545 is shown in FIGS. 1A-1C and 2A-2D.
First, a conventional developing device will be described with reference to FIGS. 1A-1C, which include conceptual drawings of a developing device having a plurality of screw augers. FIG. 1A is a cross-sectional view taken along a line A-A′ in FIG. 1B of a developing device 40. FIG. 1B is a side view in a direction X shown in FIG. 1A revealing the interior of the developing device 40. FIG. 1C is an explanatory diagram illustrating the direction in which developer is conveyed in the developing device 40. The conveying direction for developer is indicated by arrows in FIG. 1C.
As shown in FIG. 1A, the developing device 40 includes a magnetic developing roller 80; and three screw augers 90a, 100a, and 110a. Three screw auger sections 90, 100, and 110 are formed in the developing device 40 and isolated from each other by a partitioning wall 150.
The first screw auger section 90 and the second screw auger section 100 are juxtaposed vertically with the first screw auger section 90 on the top and the second screw auger section 100 on the bottom, and the third screw auger section 110 is disposed between the first screw auger section 90 and second screw auger section 100 with respect to the vertical. A drive source (not shown) produces a driving force for rotating the screw augers 90a, 10a, and 110a so that the screw augers 90a, 100a, and 110a agitate and convey developer.
The screw auger 90a agitates the developer while conveying the developer toward the magnetic developing roller 80. The sleeve of the magnetic developing roller 80 rotating clockwise in FIG. 1A holds the developer supplied from the screw auger 90a with magnetic force, forming a magnetic brush.
Developer not supplied to the magnetic developing roller 80 is conveyed to a downstream position in the first screw auger section 90 (in the left of FIG. 1B), and drops down into the second screw auger section 100 through a third opening N30.
Developer used in development and therefore having a decreased toner concentration is also recovered from the magnetic developing roller 80 by the second screw auger 100. The second screw auger 100 agitates and conveys the developer toward a downstream position (the left of FIG. 1B).
After a fixed amount of developer is accumulated in this downstream position of the second screw auger section 100, the developer migrates diagonally upward to the third screw auger section 110 through a first opening N10.
Developer that has migrated to the third screw auger section 110 is agitated and conveyed by the screw auger 110a together with new toner supplied through a supply opening N40. The screw auger 11a conveys this developer to a downstream position (right in FIG. 1B).
Next, another conventional developing device will be described with reference to FIGS. 2A-2D, which are a conceptual drawing of a developing device having a plurality of screw augers. In FIGS. 2A-2D, like parts and components to those in FIGS. 1A-1C have been designated with the same reference numerals to avoid duplicating description.
FIG. 2A is a center cross-sectional view of the developing device 40 taken along a line B-B′ shown in FIG. 2D. FIG. 2B is a cross-sectional view along a line C-C′ in FIG. 2D through one end of the developing device 40. FIG. 2C is a cross-sectional view along a line D-D′ in FIG. 2D through another end of the developing device 40. FIG. 2D is a side view along a direction X shown in FIG. 2A revealing the interior of the developing device 40.
In this conventional developing device, a screw auger 110a is oriented so that the downstream end is positioned higher than the upstream end in the direction for conveying developer in the third screw auger section 110 (see FIG. 2B).
In the developing device 40 shown in FIGS. 1A-1C and 2A-2D, the second screw auger section 100 recovers developer having a depleted toner concentration following development. The recovered developer is replenished with new toner, since some of the toner has been consumed for developing a latent image, and is reused for development. In order to replenish the new toner, a toner concentration sensor for detect the toner concentration is disposed in the third screw auger section 110.
However, in the conventional structures described above, when performing high-density printing in which a large amount of toner is consumed during development, the toner concentration drops remarkably. Therefore, it is necessary that a large amount of toner is provided to the third screw auger section 110. In such a case, since the recovered developer and the new toner are agitated only in the third screw auger section 110, they are not agitated sufficiently. Thus, the toner concentration in the developer detected by the toner concentration sensor becomes irregular, and consequently the density of the image formed on the sheet becomes unstable.